Lipid nanoparticles (LNPs), liposomes or lipoplexes are effective drug delivery systems for biologically active compounds such as therapeutic proteins, peptides or nucleic acid based therapeutics, which are otherwise cell impermeable. Liposomal formulations have also been developed for small molecule drugs with the aim to enrich the drug in certain tissues.
Drugs based on nucleic acids interact with a messenger RNA or a gene and have to be delivered to the proper cellular compartment in order to be effective. In particular double stranded nucleic acids, for example double stranded RNA molecules (dsRNA) such as siRNAs, suffer from their physico-chemical properties that render them impermeable to cells. Upon delivery into the proper compartment, siRNAs block gene expression through a highly conserved regulatory mechanism known as RNA interference (RNAi). Typically, siRNAs are large in size with a molecular weight ranging from 12-17 kDa, and are highly anionic due to their phosphate backbone with up to 50 negative charges. In addition, the two complementary RNA strands result in a rigid helix. These features contribute to the siRNA's poor “drug-like” properties (Nature Reviews, Drug Discovery 2007, 6:443). When administered intravenously, the siRNA is rapidly excreted from the body with a typical half-life in the range of only 10 min. Additionally, siRNAs are rapidly degraded by nucleases present in blood and other fluids or in tissues, and have been shown to stimulate strong immune responses in vitro and in vivo (Oligonucleotides 2009, 19:89).
By introduction of appropriate chemical modifications stability towards nucleases can be increased and at the same time immune stimulation can be suppressed. Conjugation of lipophilic small molecules to the siRNAs improves the pharmacokinetic characteristics of the double stranded RNA molecule. It has been demonstrated that these small molecule siRNA conjugates are efficacious in a specific down regulation of a gene expressed in hepatocytes of rodents. However, in order to elicit the desired biologic effect a large dose was needed (Nature 2004, 432:173).
With the advent of lipid nanoparticle formulations the siRNA doses necessary to achieve target knockdown in vivo could be significantly reduced (Nature 2006, 441:111). Typically, such lipid nanoparticle drug delivery systems are multi-component formulations comprising cationic lipids, helper lipids, lipids containing polyethylene glycol and cholesterol. The positively charged cationic lipids bind to the anionic nucleic acid, while the other components support a stable self-assembly of the lipid nanoparticles.
To improve delivery efficacy of these lipid nanoparticle formulations, many efforts are directed to develop more appropriate cationic lipids. These efforts include high throughput generation of cationic lipid libraries based on solvent- and protecting group free chemical reaction such as Michael additions of amines to acrylamides or acrylates (Nature Biotechnology 2008, 26:561) or ring-opening reactions with amines and terminal epoxides (PNAS 2010, 107:1854). Another strategy involves structure activity studies, e.g. systematic variation of the degree of saturation in the hydrophobic part (Journal of Controlled Release 2005, 107:276) or the polar head group of the cationic lipid (Nature Biotechnology 2010, 28:172), resulting in an improved efficacy of the so-called stable nucleic acid-lipid particles (SNALP) technology (Current Opinion in Molecular Therapeutics 1999, 1:252).
Despites these efforts, improvements in terms of increased efficacy and decreased toxicity are still needed, especially for lipid nanoparticle based drug delivery systems intended for therapeutic uses. LNPs naturally accumulate in the liver after intravenous injection into an animal (Hepatology 1998, 28:1402). It has been demonstrated that gene silencing can be achieved in vivo in hepatocytes which account for the majority of the cells in the liver. Even the simultaneous down-modulation of several target genes expressed in hepatocytes could be successfully achieved (PNAS 2010, 107:1854). However, evidence of successful gene regulation in other liver cell types is lacking.